502 . CHEMICAL ENGINEERING kind of material. Where the time of exposure to the liquid mass is but short the comparatively low refractoriness of the lining is not of great significance. The mechanical strength of the plastic fireclay bricks is high and frequently the resistance to compression attains 5,000 Ib. per square inch and more. Grog Refractories.—In ceramic usage the term "grog" implies calcined clay or broken and ground waste product, rejected during the course of manufacture due to some defect like cracking, or from material which has already been in use, such as broken firebricks, glass pots, etc. The grog refractories, composed of burnt and plastic clay, generally in the proportion of 1:1, constitute a large and important class of materials, especially in the glass industry. In this class of bodies it is necessary to consider not only the character of the clay but that of the grog as well, together with the coarseness or fineness of the latter, expressed by its surface factor, viz., 2) a\/d\ + a2/dz + az/ds + . . . , where ai, at, etc/, equal percentage by weight of each group of sizes, obtained by separation through sieving, and di, d2, d3, etc., equal diameter of each group, determined by means of the micrometer microscope or other methods. The factor thus expresses the . total surface of the grains and hence the fineness, and rapidly becomes larger for particles which pass the 80-mesh and finer sieves. It obviously is the more accurate the closer the size groups approach each other, but for practical purposes sizing between the 8 to 20 mesh, 20 to 40,40 to 60,60 to 80,80 to 120, and through the 120-mesh sieve, is sufficient to establish the surface or fineness factor. The finer the particles are the more they will unite with the bond clay, thus increasing its refractoriness if the grog is more refractory, or decreasing it if the latter is deficient in this respect. The finer the grog, the more plastic will be the fired body mixture and the denser and stronger the resulting product. The strength in the dry state increases with grog down as fine as that passing the 100-mesh sieve, but decreases with material passing the 200-mesh sieve. For bodies which are to be strong, dense and resistant to the action of fluxes, fines passing the 80-mesh sieve are necessary up to about 50 per cent, but for products which must resist the shocks of sudden temperature changes the fine material must be kept at about 25 per cent. Clays high in clay substance as well as siliceous ones are used as bonding materials for grog mixtures. Thus for highly refractory muffles, boiler tiles, etc., a clay of the first type is employed; for zinc muffles, etc., one of an intermediate silica content; while for glass pots, tank blocks, etc., either a mixture of the first and a siliceous clay is preferred or the exclusive use of the last type. A discussion of glass house refractories may bring out facts applicable to other grog bodies. Glass Pot Mixtures.—It has been assumed by American glass manufacturers that the refractories used by them, glass pots, tank blocks, floaters, boots, etc., for best results require the use of German clay, principally that obtained at Grossalmerode, the properties of which are discussed in preceding pages. The fact that this clay possesses good plasticity, safe drying qualities, low drying and low fire shrinkage, and at the same time, owing to its siliceous nature, resists the action of the commercial glasses very satisfactorily, has made it a standard raw material. The low temperatures employed in American pot arches have required the use of clays of low fire shrinkage, since evidently it is not admissible to permit the contraction taking place in the melting furnace to be too great, owing to the severe stresses to which the pots would be subjected.